A method and such a device are known from WO 2005/084995 A1.
A conventional motor vehicle with an internal combustion engine can become slower due to the engine drag torque when a driver takes their foot off the accelerator pedal. In the case of a relatively severe negative gradient the engine drag torque can be insignificant. This also depends on the mass of the vehicle, the current vehicle speed and the currently engaged gear speed.
In motor vehicles with an internal combustion engine and automatic transmission there is what is commonly referred to as a “coasting operating mode” in which the drive train is automatically opened if the driver takes their foot off the accelerator pedal. The internal combustion engine can be idled in the coasting operating mode or switched off entirely, for example, in the case of an automatic start/stop system. In the coasting operating mode, the engine drag torque is eliminated in order to save fuel.
Various driver interactions such as, for example, propulsion requests or braking requests through activating the accelerator pedal, brake pedal or activating the gear speed selector lever can cause the drive train to close, i.e. caused the coasting operating mode to terminate. With this general logic, a driver has control over the activation and deactivation of the coasting operating mode and they can choose between the coasting operating mode and rolling with the engine drag torque.
It is to be expected that drivers adapt their driving style to the presence of the coasting operating mode. This means that the driver does not expect the vehicle to become significantly slower if the driver takes their foot off the accelerator pedal. However, the vehicle behavior in the coasting operating mode is different if the road on which the vehicle is currently driving has a negative gradient or a negative gradient occurs further along the route. In particular, in the coasting operating mode the vehicle becomes faster on a moderate negative gradient.
In order to solve this problem, the abovementioned document WO 2005/084995 A1 proposes deactivating a currently activated coasting operating mode automatically even when the current vehicle speed exceeds, or significantly exceeds, a threshold value. This threshold value may be a value which is preset by the driver or set automatically as a function of the current negative gradient and/or of a negative gradient lying ahead. The current negative gradient or negative gradient lying ahead is to be determined using satellite navigation.
Determining the current negative gradient or a negative gradient lying ahead using satellite navigation is complex.
One possibility is to consider calculating the vehicle acceleration in the direction of travel either by differentiating the vehicle speed or measuring it directly and automatically deactivating a coasting operating mode if the calculated or measured vehicle acceleration exceeds a threshold value. However, differentiation of the vehicle speed can result in a noisy acceleration signal.